Trough collector for a solar power plant

ABSTRACT

A trough collector ( 1 ) for solar power plants comprises a pressure cell ( 25 ) containing an absorber pipe ( 42 ) for a heat-carrying fluid and also a secondary concentrator which is likewise arranged in the pressure cell ( 25 ). The pressure cell ( 25 ) thus can be reduced in height, which eliminates the need for reinforcements in the form of a framework structure for the pressure cell ( 25 ), which are otherwise required.

The present invention relates to a trough collector for a solar power plant according to the preamble of claim 1.

For some time, solar thermal power plants have already been producing power on an industrial scale at prices which, compared to photovoltaics, are close to the commercial prices now usual for power produced in the conventional manner.

In solar thermal power plants, the radiation of the sun is reflected using the concentrator through collectors and focused in a targeted manner on a location in which high temperatures arise as a result. The concentrated heat can be conducted away and used to operate thermal engines such as turbines which in turn drive the generators which generate electricity.

Three basic forms of solar thermal power plants are currently in use: dish/Stirling systems, solar tower power plant systems and parabolic trough systems.

Parabolic trough power plants have a large number of collectors which have long concentrators having small transverse dimensions and therefore do not have a focal point but a focal line, which fundamentally distinguishes these in their design from the dish Stirling and solar tower power plants. These linear concentrators today have a length of 20 m to 150 m whilst the width can reach 3 m, 5 m and more. An absorber element for the concentrated heat (up to around 400° C.) runs in the focal line, which transports this to the power plant. A fluid such as, for example, thermal oil or superheated water vapor which circulates in the absorber elements can be considered as transport medium.

Although a trough collector is preferably constructed as a parabolic trough collector, trough collectors with a spherical or only approximately parabolically configured concentrator are frequently used since an exactly parabolic concentrator having the aforesaid dimensions can only be produced at great expense, which is therefore hardly economically reasonable.

The 9 SEGS trough power plants in Southern California together produce a power of about 350 MW; an additional power plant in Nevada should presently be going onto the grid and deliver over 60 MW. A further example of a trough power plant is the Andasol 1 under construction in Andalusia having a concentrator area of 510,000 m² and 50 MW power, where the temperature in the absorber elements should reach about 400° C. The pipeline system for circulating the heat-transporting fluid in such power plants can reach a length of up to 100 km or more if the concepts for future large plants are implemented. The costs for Andasol 1 are estimated at several hundred million Euros.

According to rough calculations, it can be determined that an increasingly large proportion of the overall costs, today for example 65% or more in such a solar power plant, are attributed to the trough collectors and the pipeline system for the heat-transporting fluid.

In view of the said high building costs and also the high maintenance costs for such trough collectors, designs were even produced early on in which the contamination of the reflecting concentrator surface is reduced and thus the maintenance expenditure, but also the expenditure for the construction of the collector itself is reduced.

FR-PS 1,319,144 thus discloses a tub or trough collector for a solar power plant of the aforesaid type which has a cylindrical pressure cell comprising a flexible membrane which has a transparent region and a reflective region. Due to increased internal pressure, the cylindrical pressure cell is held in its shape so that solar radiation incident through the transparent region reaches the spherically curved reflective region and from there is concentrated onto an absorber element running in the pressure cell.

With such an arrangement it is fundamentally feasible to heat an absorber element running through the cylindrical pressure cell along its length, this being protected by the pressure cell from contamination and cooling by the wind. The cleaning expenditure thus eliminated for the absorber element and especially the protection from cooling are cost factors which should not be underestimated in the production of solar power on an industrial scale.

A conventional absorber element, for example, running in the open air, loses up to 100 W/m due to heat emission and cooling by the ambient air, which corresponds to a loss of 10 MW in the case of a line length of up to 100 km (or more). Any reduction in this loss, for example, by protection from wind cooling is therefore important and is to be striven for.

The construction of the concentrator as a flexible membrane having a low weight further offers the possibility of constructing the entire supporting structure of the collector more simply and therefore more favorably since, compared with the conventional solid mirrors, substantially lower weight needs to be mounted and pivoted (in accordance with the position of the sun). Finally, a concentrator consisting of a flexible membrane is also more favorable to produce and easier to bring to the building site (which by the very nature of the matter can be located in remote areas, e.g. a desert) and can also be mounted more easily than is the case with conventional concentrators.

However, the construction of a trough collector with a cylindrical pressure cell is not practicable with the aforementioned dimensions required today. The extremely large working surface for wind, for example formed by the cylindrical membrane body, requires reinforcement of the cylindrical pressure cell by a solid framework such as already mentioned in the said French patent specification with regard to reinforcing “nervures annulaires” of the cylindrical body. Such a framework which is additional to the supporting structure for the cylinder, which is inherently lighter and simpler to construct, loses the disclosed embodiment the advantages initially mentioned in the French patent specification relating to the lighter and more favorable construction to a significant extent.

FR-PS 2 362 347 shows the same design of a trough collector with inflatable collector body. Here it is taken into account that solar radiation incident on the transparent section of the membrane is reflected partially as a function of the angle of incidence; in practice, not when the solar radiation is incident perpendicularly onto the transparent section and with increasing proportion, the more sloping the incidence.

A cylindrical pressure cell accordingly has the property that (when viewed in the direction of the solar radiation) solar rays incident thereon at the edge are substantially reflected, i.e. only a small fraction reach the concentrator.

Accordingly, the cylindrical trough collector presented in FR-PS 2 363 347 has a reflecting layer which does not extend over the inherently possible half of the cylinder jacket but only over a segment of this cylinder half.

As a result, for the same concentrator area, an even larger cylinder diameter is obtained than in the design according to FR-PS 1 319 144, which is correspondingly unfavorable. Assistance is sought by “cutting away” to a certain extent the jacket regions running parallel to the solar radiation in the cylindrical body; a biconvex, lens-shaped structure is thus obtained, whose thickness or height is reduced compared with the diameter of the cylinder with the same fraction of incident or non-reflected sunlight.

Instead of the biconvex structure, a structure may also occur, in which the transparent membrane is replaced by means of a fixed (transparent) plate which is oriented perpendicularly to the incident sunlight, with the advantage that practically no more solar radiation is reflected, and with the disadvantage that the costs for the production of such a trough collector increase, as the advantages in the case of the use of a membrane (lightweight construction which also allows more beneficial structures for the framework of the collector, etc.) are lost again at least to some extent.

In addition, the person skilled in the art identifies that the disclosed embodiments also in biconvex i.e. lens-shaped configuration, actually still have considerable height since the focal line region lies inside the pressure cell and thus the radius of curvature of the concentrator is correspondingly small, which, in the case of a trough collector of 5 or 10 m width, leads to a considerable volume of the pressure cell. The working surface even for laterally incoming wind (for example, for strong wind in the rest position) is as previously so large that here also an additional framework which stabilizes the pressure cell under wind action must be added to the supporting structure for the pressure cell itself, i.e. cannot be omitted.

The supporting structure for the pressure cell can substantially consist of a simple rectangular frame in which the pressure cell is spanned along its longitudinal edges.

WO 90/10182 (which corresponds to U.S. Pat. No. 5,365,920) shows a trough collector in FIG. 1, which should be reduced in terms of height compared to the cylindrical configuration, but should nonetheless allow a larger extension of the spherically curved concentrator 2. This large extension for high concentration of the radiation means that the distance of the absorber pipe 6 is not much smaller than the radius of curvature of the concentrator 2 in cross section. On account of the spherical curvature of the concentrator 2, the focal line region is in turn extended in such a manner that radiation, which can only be concentrated insufficiently by means of the concentrator 2, has to be “reconcentrated”, which takes place with the aid of a secondary concentrator 5 which is cylindrical according to the disclosure mentioned. The person skilled in the art recognizes easily however that this is virtually not possible by means of a cylindrical secondary concentrator 5: it is true for reflection at the secondary concentrator 5 that the angle of incidence of the radiation is the same as the emergent angle, which angles are measured relatively to the perpendicular onto the respective surface region of the secondary concentrator 5. This perpendicular is then dropped from the longitudinal axis of the absorber pipe 7 upon the surface of the secondary concentrator. It is therefore true for all rays reflected at the concentrator 2, which miss the absorber pipe 7 by a distance on account of the large focal line region (and therefore make it onto the secondary concentrator 5), that they must also in turn miss the same by the same distance following their reflection at the cylindrical secondary concentrator. The person skilled in the art will therefore barely be able to realize the arrangement shown in FIG. 1 and therefore the preferred embodiments which do not require a secondary concentrator according to the disclosure mentioned (page 9). These embodiments show a pressure cell with a sufficiently large radius of curvature of the concentrator 54. A pressure cell of this type thus has the desirably advantageously small height, the absorber pipe 50 having to be arranged well outside of the pressure cell, however.

DE OS 27 33 915 (which corresponds to FR PS 2 398 982) shows a conventional trough collector with rigid concentrator (that is to say without a pressure cell) which is constructed in accordance with the object to hold the concentrator in a correct orientation to the sun and to prevent a cooling down of the absorber pipe (during the night). To this end, a secondary concentrator is arranged in the focal line region behind the focus f of the concentrated radiation, which reflects the concentrated radiation onto an absorber pipe which is fixed with respect to the pivoting movement of the concentrator. The fixed absorber pipe can then be closed off from the outside world overnight by means of doors, so that a heat loss is prevented. As a pressure cell is not present, a reduction of the height of the arrangement is not necessary. The height or the distance of the focal line region and therefore also of the secondary concentrator from the concentrator corresponds to the height or distance of the absorber pipe 50 (according to the above-mentioned WO 90/10182) from the concentrator of the pressure cell shown there.

Accordingly, the solar power plants of the type with trough collectors which are now being built or planned possess such a conventional type without a pressure cell and therefore do not achieve the inherently feasible advantages (cost-effective lightweight construction, simplified maintenance due to concentrators protected from contamination, no cooling of the absorber element by wind etc.).

Thus, it is the object of the present invention to provide a trough collector with a pressure cell, which can be realized in the case of conventional widths, but also at a width of 8 m, 10 m or more.

To achieve this object, the trough collector according to the invention has the features of claim 1.

Since the concentrator concentrates the incident solar radiation toward a focal line region running outside the pressure cell, it can be designed in a flat manner, i.e. having a large radius of curvature which provides the basis for keeping the height of the pressure cell small; since a secondary concentrator is provided in the path of the firstly concentrating radiation, that is to say upstream of the focal line region, the absorber element can be arranged in the pressure cell at a suitable location without increasing its height, with the consequence that the pressure cell, for example, can now be built without its own reinforcing framework and thus its advantages as an inexpensive lightweight construction solution can be utilized.

Beyond the object set, a production method for such a secondary concentrator is specified, which does not have to rely on experiments and is also suitable as an instrument for the person skilled in the art to design the secondary concentrator for a collector quickly and simply in the actual case and thus to be able to produce it, designed in a correct, improved or corrected manner, even on site if necessary. The latter can also be considered for retrofitting conventional collectors.

The method according to the invention allows a simple calculation of the curvature of the reflective surface of the secondary concentrator which is not complex, with the accuracy desired in each case, so that the secondary concentrator can easily be correctly designed and produced in the actual case according to the design of a trough collector.

Preferred embodiments have the features of the dependent claims.

The invention is explained in more detail on the basis of the figures. In the figures:

FIG. 1 schematically shows a view of the trough collector according to the invention,

FIG. 2 shows a cross section through the trough collector of FIG. 1 with a first arrangement of the secondary concentrator

FIG. 3 schematically shows a cross section through the trough collector of FIG. 1 with a second arrangement of the secondary concentrator,

FIG. 4 shows a preferred embodiment of the trough collector according to the invention,

FIG. 5 shows a cross section through the pressure cell of a trough collector with two pressure chambers,

FIG. 6 shows a cross section through an absorber element, and

FIG. 7 shows a co-ordinate system placed into the arrangement of FIG. 4 with the vectors used in the method according to the invention.

FIG. 1 shows a view of the trough collector 1 according to the invention, with a supporting structure 2 consisting of a rectangular frame 3 which is mounted on a pivoting device 4 which for its part rests on suitable supports 5. The pivoting device 4 allows the trough collector 1 to be aligned according to the respective position of the sun. The rectangular frame 3 is preferably constructed from concrete, which is beneficial for a simplified production on site, that is to say on the building site itself.

The figure further shows a pressure cell 10 under operating pressure, i.e. in the inflated state with the cushion-like shape symbolized by the auxiliary lines 11. The length of such a trough collector 1 can exceed 250 m, the width can exceed 10 m. The height of the pressure cell 10 is dependent on the width but in any case, is smaller than this.

An absorber pipe 12 is heated by the reflected solar radiation; the concentrator is situated in the pressure cell 10 and is therefore not visible in the figure. The absorber pipe 12 is mounted on suitable supports 13 so that it is situated in the focal line region of the concentrator.

In the following figures, the same reference numerals are used for the same parts.

FIG. 2 schematically shows a cross section through a first embodiment 20 of the trough collector 1. The pivoting device 4 is formed by an arcuate pivoting bracket 6 which runs in a pivoting drive 7, which adjusts the pivoting bracket 6 by means of rollers 8 and thereby aligns the frame 3 in the desired position.

Mounted in the frame 3 (and therefore tensioned by said frame) is a pressure cell 25 which is formed from flexible membrane sections 26,27, in the present case two thereof, which are connected along their edges 28,29. The one, in this case, upper membrane section 26 is at least partially transparent for incident sunlight, shown by the rays 30, 31; the other, lower membrane section 27 is formed by a reflecting coating 32 facing the sun's rays 30,31 as concentrator 33.

Under operating pressure, the membrane sections 26,27 are acted upon by pressure and thereby spherically curved so that the concentrator 33 adopts its trough shape in the frame 3, which has given this genre of solar collectors their name.

Due to the spherical curvature, it is found that compared with an exact parabolic curvature, the rays 30′, 31′ reflected by the concentrator 33 are concentrated into a focal line region 34 and not into a focal line. The focal line region 34 is indicated schematically by the dotted line. The design of a trough collector 20 to be constructed, i.e. length, width etc. i.e. the design of the radius of curvature 36, is made by the person skilled in the art with regard to a specific project.

The radius of curvature 36 of the concentrator 33 is selected to be sufficiently large that the focal line region 34 lies outside the pressure cell 25 with the advantage that this is configured to be flat, i.e. has a small height. In the case of strong winds, the pressure cell 25 can be aligned by the pivoting drive 7 in such a manner that it is only exposed to the wind action from the narrow side, having its small height. Thus, an additional framework for stabilizing the pressure cell 25 (or other design measures stabilizing the pressure cell 25) are omitted. The pressure cell 25 with the concentrator 33 has a low weight compared with a conventional solid-structure concentrator with the consequence that the frame 3 and also the pivoting device 4 with the supports 5 can have smaller dimensions. As a consequence of this lightweight structure, not only a cost reduction is achieved on the material side but also substantially simplified production and assembly which is particularly suitable for remote areas such as desert areas.

Further provided in the pressure cell 25 are a secondary concentrator 40 and an absorber element 42 which are suspended on retaining plates 45 of the frame 3 by means of a supporting framework with supports 44 indicated by dashed lines. Despite the large radius of curvature of the concentrator 33, the advantage thus still exists that the solar radiation 30,31 does not have to pass through the transparent membrane 26 twice; with the consequence that the loss due to aging of the membrane 26 also remains correspondingly small: per percent of loss of transparency of the membrane 26 (for example contamination or scratching of the surface) a doubled energy loss of two percent in each case would result in the case of a double passing of the solar radiation 30, 31 through the membrane 26.

The construction and dimensioning of the above-mentioned supporting framework can easily be made by the person skilled in the art for a trough collector 1 to be specifically constructed.

A heat-transporting fluid which removes heat produced by the incident concentrated radiation 30, 31 circulates in the absorber element 42 in a known manner.

The maintenance expenditure for the secondary concentrator 40 and the absorber element 42 is small since both (like the concentrator 33 itself) are protected from contamination by the pressure cell 25. This can additionally be filled with, for example, nitrogen instead of the ambient air, which additionally prevents or slows corrosion of the reflective layer of the concentrator 33 and also the secondary concentrator 40 (if this should consist of sheet metal, for example, see below). Alternatively, the air in the pressure cell 25 can be dehumidified.

At the same time, the secondary concentrator 40 is arranged in the path of the radiation 30′, 31′ which is being concentrated by the concentrator 33 and is constructed in such a manner that the concentrated radiation 30′, 31′ is further concentrated toward a secondary focal line region 41 at the location of the absorber element 42. An arrangement is obtained in which the secondary concentrator 40 is located opposite to the concentrator 33 and close to the transparent membrane section 26, the absorber element 42 running between the concentrator 33 and the secondary concentrator 40. Since the curvature of the concentrator 33 is weakly defined (large radius of curvature 36) for the given width of the trough collector 1, the focal line region 34 has only a small extent with the advantage that the secondary concentrator 40 has a narrow structure and casts only a few shadows.

The absorber element 42 is protected from external influences such as wind and accordingly loses little heat in operation.

The basic layout shown in the figure for a trough collector according to the present invention is symmetrical with respect to a plane of symmetry running centrally along the trough formed by the concentrator 33, this line being indicated by the dot-dash line 43.

FIG. 3 shows a second embodiment of the trough collector 1 according to the invention. A support structure 50 divides the concentrator 51 into a plurality, here two, of longitudinally running regions 52,53 which are inclined in a v-shaped manner with respect to one another and, as they are acted on by pressure, are each curved spherically. The concentrated solar rays 30′31′ each fall onto an associated region 55,56 of a correspondingly constructed secondary concentrator 54 and make it, as rays 30″,31″ further concentrated by means of this, to the absorber element 42. The concentrator 51 and the secondary concentrator 54 are therefore each divided into two longitudinally running mutually symmetrical regions 52,53 and 55,56.

In the case of a not substantially enlarged structural height of the pressure cell 25, this arrangement has the advantage that only little or no shade falls onto the concentrator surface 52,53 and the absorber element 42 itself is illuminated by means of solar radiation.

FIG. 4 shows a further preferred embodiment of the collector 1 according to the invention. The concentrator 60 has two longitudinally running reflective regions 61,62 which enclose between them a central strip 63 which advantageously belongs to the supporting structure of the trough collector and is therefore walkable. Thus, the absorber element 64 running over the central strip 63 is easily accessible for assembly and maintenance in the same way as the secondary concentrator 65 which likewise runs over the central strip 63 and is arranged under the transparent membrane section 26, which secondary concentrator for its part possesses two regions 66,67 which each further concentrate the solar radiation 30′,31′ from the region 61,62 of the concentrator 60 allocated to them towards the absorber element 64. This arrangement has an unchanged small overall height compared with that of FIG. 2, is favorable for assembly and maintenance but is additionally non-sensitive to the shadow casting of the secondary concentrator, since the two regions 61,62 of the concentrator 60 cannot be reached or only marginally reached by shadows because they are separated from one another by the central strip 63.

It is fundamentally desirable to configure the focal line region to be as small as possible, which leads to a high temperature and therefore to an improved efficiency of the thermal machines of the solar power plant. On the other hand, in a focal line region which approaches the ideal focal line, temperatures can be produced which cause a conventional absorber element consisting of metal to glow locally.

The temperature distribution at the absorber element 42,64 is substantially also determined by means of the design of the secondary concentrator 40,54,65, as the concentrator, as a membrane which is acted on by pressure, is always curved spherically. A desired temperature distribution at the absorber element 42,64 in the actual case is achieved by means of corresponding curvature of the secondary concentrator 40,54,65 or its regions 55,56 or 66,67 transversely to the length. The person skilled in the art can easily determine a suitable profile of the curvature for example by means of experiments. So, a small laser lamp can for example be moved parallel on a rail (which simulates the rays of the sun) and the focal line region of the concentrator can be represented by means of a reflective narrow strip (with the curvature of the concentrator) set up perpendicularly to the ray by means of parallel displacement of the lamp. At the location of the secondary concentrator, there is in turn a second reflective narrow strip (likewise arranged perpendicularly to the ray), the curvature of which can be changed continuously until the desired result for the secondary focal line region has been reached or a focal line has been created.

Here, it is to be added that the ideal focal line fundamentally cannot be created because of the diameter of the sun. Beyond the set object, in the case of the configuration according to the invention by means of the use of a secondary concentrator 40,54,65 in the concentrating ray 30′,31′, the considerable advantage results that the light path between the concentrator 33,60,65 and the absorber element 42, 64 is shorter than would be the case in a comparable configuration without secondary concentrator 40,54,65; thus, the defocussing is smaller and a focal line better approaching the theoretical one can be achieved.

The secondary concentrator 40,54,65 advantageously consists of a profile made of sheet metal (or of two such profiles which then form the regions 55,56 or 66,67 of the secondary concentrator) which is elongate, and curved transversely to its length and has a reflective layer on its concave side. The person skilled in the art then determines (as mentioned above, optionally by experiments) the desired curvature of the profile so that the desired focal line region results in the predetermined relative position of concentrator 33,60,65 and secondary concentrator 40,54,65. Alternatively, depending on the construction of the absorber element 64 (see below, FIG. 6), a focal line can naturally also be striven for instead of a focal line region.

The predetermined curvature can also be approximated by means of a number of straight sections which follow the profile of the curvature; if a sheet metal profile is used, the profile shape to be produced is to be produced easily by means of single or multiple beveling.

On the other hand, the desired profile shape can easily be produced by shaping sheet metal in a press which allows the focal line region (or the focal line) to be defined precisely. The reflective layer can be applied to the sheet metal or to the concave side of the sheet metal profile produced before or after the shaping process.

In other words it is the case that the secondary concentrator 40,54,65 also has a simple and robust structure and has a comparatively low weight; furthermore, it is also easily transportable and can optionally be produced on site.

FIG. 5 shows a cross section through a further embodiment of a pressure cell 70 for a trough collector 1, which is subdivided by the concentrator 71 into a first pressure chamber 72 and a second pressure chamber 73. A side 74 of the concentrator 71 reflecting the solar radiation 30,31 faces the transparent region of the membrane section 26 which is transparent at least to some extent. The figure further shows a side 75 of the concentrator 71 which faces away from the solar radiation. In the first pressure chamber 72, a higher pressure prevails than in the second pressure chamber 71, so that the concentrator is spherically curved under operating pressure and the radiation 30, 31 concentrates onto the secondary concentrator.

The advantage of this arrangement consists in the fact that the operational pressure difference in the pressure chambers 72,73 can be kept very small, which allows the concentrator 71 to be constructed in a correspondingly thin-walled manner and thus a reflective coating of very high quality to be provided. By contrast, the membrane sections 26 and 75 can be constructed robustly.

In the first pressure chamber 72, the operating pressure is applied by means of a fluid line which is preferably constructed as an air line 77, wherein a fan is preferably provided as fluid pump. The second pressure chamber 73 is fed via an overflow line 79 which connects the two pressure chambers 72,73 to one another. The (small) pressure difference between the pressure chambers 72,73 is generated by means of a further fluid pump constructed as a fan 80. It is less than 0.5 mbar, is preferably in a range from 0.05 to 0.2 mbar, and particularly preferably in the range from 0.05 to 0.1 mbar.

By these means, pressure is operatively generated and maintained at a predetermined level in each of the pressure chambers 72,73 if the volume or one or both pressure chambers 72,73 is changed in operation by means of external influences such as for example wind. This is in particular the case if, due to gusts of wind, the curvature of the membrane section 26 or 26 is pushed in or deformed in another manner. By means of the means provided, the pressure difference always remains essentially constant so that the concentrator of thin-walled construction keeps its spherical shape and also cannot be overloaded: if the volume of the pressure chambers becomes smaller, the enclosed air can also flow out quickly against the operating direction of the fans if necessary. This configuration with coupled fans 78,80 expands the beneficial properties of the pressure cell 25 according to the invention, that is to say allows the use of a concentrator with a high-quality reflective coating without the pressure cell having to be supported by means of its own framework (or other measures). A pressure cell 70 with two pressure chambers 72,73 of the type mentioned is described in the same applicant's PCT/CH2007/000480, the content of which is hereby incorporated into this application, particularly with regard to the executability and the protective scope of the present invention.

FIG. 6 shows an absorber element 70 with a jacket 77 which reduces the heat emission to the outside, but has a thermal opening 72 running longitudinally in it, which allows the passage of solar radiation 30″,31″ concentrated by the secondary concentrator 40,54,65 through the jacket 77 into the interior 73 of the absorber element 70. As a result, the concentrated heat of the absorber element is supplied from the inside so that the jacket 77 can be insulated simply and faultlessly. The fluid transporting the heat circulates in the annular space 75 formed between the interior 73 and the jacket 77. The interior wall 76, which is preferably constructed in a wave-shaped manner, scatters locally incident solar radiation by means of multiple reflection diffusely onto its entire surface until it is completely absorbed.

This construction allows an improved insulation against heat loss compared to absorber elements of the prior art and in this regard expands the advantages which result by means of the present invention. If the curvature of the secondary concentrator is matched to a thermal opening 72 which is as small as possible, an optimal heat usage results by means of a trough collector according to the present invention.

FIG. 7 then shows an x-y co-ordinate system placed in the arrangement of FIG. 4, with the vectors used in the method according to the invention, with the co-ordinate system lying in a cross-sectional plane, in this case of the secondary concentrator 65. Of course also placed in a cross section through the trough collector 20 (FIG. 2) or in a collector of any desired construction.

The central strip 63 is indicated in FIG. 7 for the purpose of illustration but does not play any role in the calculation process described below. The given concentrator 32, 62 can be seen, which has a circular arc shape in the illustrated cross section because it is acted on by pressure in the present case and for which a secondary concentrator 40,65 is to be determined. Further visible is the solar ray 31, the solar ray 31′ which is reflected on the concentrator 32,62, and the solar ray 31″ which is in turn reflected (shown using dashed lines) on the secondary concentrator and is incident on the absorber pipe 42,64, which is indicated using dash-dotted lines, and is absorbed by the latter. In the figure, the absorber pipe 42,64 is shown by the focal point or a point on the focal line F(0;y_(φ)). The absorber pipe 42,64 is thus predefined in the same manner as the concentrator 32,62, that is, for the calculation process.

In the secondary concentrator 40,65 production method steps described below, vectors are indicated as such by underlining.

C(x_(c);y_(c)) denotes the curvature center point of the predefined concentrator 32,62. PM(φ) denotes a general point on the concentrator 32,62 (or on the concentrator surface); R_(M) denotes the radius of curvature of the concentrator 32,62 which runs through the point PM(φ), where φ is the angle of R_(M) to the y axis.

In other words, all points on the concentrator 32,62 from which the solar radiation 31 is to be reflected into the absorber pipe 42,64 are given the angle φ.

The vectors s(φ), s′(φ) and s″(φ) are unit vectors in the path of the respective solar rays (and for the sake of simplicity are no longer explicitly shown below as functions of φ): s lies in the path of the solar ray 31, s′ lies in the path of the ray 31′ which is reflected on the concentrator 32,62, and s″ lies in the path of the ray 31″ which is reflected on the secondary concentrator.

R_(M) as the radius of curvature forms the perpendicular to the concentrator at the point PM(φ). Incident and reflected rays form the same ray to the perpendicular, correspondingly the angles between R_(M) and the ray 31, and between R_(M) and s′ are equal to each other and equal to φ.

The reflected ray 31′ meets the secondary concentrator 40,65 at the point SM(φ,k), with the value of k as the stretch factor for s′ being unknown.

t _(SM)(φ) is the tangent to the secondary concentrator 40,65 at the point SM(φ,k).

The unit vectors s′ and s″ form an angle 2α between them. As t _(SM) is the tangent to the secondary concentrator 40,65 at the point SM(φ,k), it is therefore also the angle bisector between s′ and s″, according to the above-mentioned principle according to which the incident and the reflected rays assume the same angle to the perpendicular. (t _(SM) as the tangent is perpendicular to the perpendicular at the point SM(φ,k)).

If the unknown factor k is now defined in such a manner that s″ goes through the focal point F (or the absorber pipe 42,64), SM(φ,k) is known. The shape of the reflective surface of the secondary concentrator 40, 65 is thus known, since each value φ (that is, each irradiated point on the surface of the concentrator) can be assigned a corresponding point on the surface of the secondary concentrator 40,65, which corresponding points together form the surface of the sought secondary concentrator 40,65.

In summary, the vector sum of the vectors C+CPM(φ)+k·s′ is equal to the vector SM(φ,k), which corresponds in the co-ordinate system to the respective point, which corresponds to an angle φ, on the reflective surface of the secondary concentrator 40,65.

In a preferred embodiment of the production method according to the invention, k is defined in that the scalar products s′·t _(SM)(φ) and s″·t _(SM)(φ) are equalized to s′·t _(SM)(φ)=s″·t _(SM)(φ), as the angle α enclosed in each case is equal. In this preferred embodiment, the calculation is presented as follows:

$\mspace{20mu} {\text{?} = {\begin{pmatrix} 0 \\ {- 1} \end{pmatrix}\mspace{14mu} {and}\mspace{14mu} {thus}}}$ $\mspace{20mu} {{{{\underset{\_}{PM}(\phi)}\text{?}} + {\text{?}\begin{pmatrix} {\sin \; \phi} \\ {{- \cos}\; \phi} \end{pmatrix}\mspace{14mu} {and}\mspace{14mu} {\underset{\_}{s}}^{\prime}\text{?}}} = \begin{pmatrix} {{- \sin}\; 2\phi} \\ {\cos \; 2\phi} \end{pmatrix}}$ ?indicates text missing or illegible when filed

The equation of the secondary concentrator 32,62, see above, has the form

whereby the tangent t _(SM)(φ) as the derivative of the expression

becomes

$\mspace{20mu} \begin{matrix} {{\text{?}(\phi)} = \frac{\text{?}}{\phi}} \\ {= {{\frac{\text{?}}{\phi} + {\frac{k}{\phi} \cdot {\underset{\_}{s}}^{\prime}} + {k \cdot \frac{{\underset{\_}{s}}^{\prime}}{\phi}}} =}} \\ {= {{\text{?}\begin{pmatrix} {\cos \; \phi} \\ {\sin \; \phi} \end{pmatrix}} + {\frac{k}{\phi} \cdot \begin{pmatrix} {{- \sin}\; 2\phi} \\ {\cos \; 2\phi} \end{pmatrix}} + {2 \cdot k \cdot \begin{pmatrix} {{- \cos}\; 2\phi} \\ {{- \sin}\; 2\phi} \end{pmatrix}}}} \end{matrix}$ ?indicates text missing or illegible when filed

s″ is:

${{\underset{\_}{s}}^{''}(\phi)} = \left. \frac{\text{?} - \text{?}}{{\text{?} - \text{?}}}\rightarrow{\begin{pmatrix} \text{?} \\ \text{?} \end{pmatrix} - \begin{pmatrix} 0 \\ \text{?} \end{pmatrix} + {\text{?}\begin{pmatrix} {\sin \; \phi} \\ {{- \cos}\; \phi} \end{pmatrix}} + {k\begin{pmatrix} {{- \sin}\; 2\phi} \\ {\cos \; 2\phi} \end{pmatrix}}} \right.$ ${\text{?}\sqrt{\left( {\text{?} + {\text{?}\sin \; \phi} - {k\; \sin \; 2\phi}} \right)^{2} + \left( {\text{?} - \text{?} - {\text{?}\cos \; \phi} + {k\; \cos \; 2\phi}} \right)^{2}}} = \sqrt{\ldots}$ ?indicates text missing or illegible when filed

The root in the denominator of the equation for s″, for short

, is no longer analytically resolved.

s″, s″ and t _(SM)(φ) are now calculated and so can be inserted into s′·t _(SM)(φ)=s″·t _(SM)(φ).

The result is that

$\mspace{20mu} \begin{matrix} {{{\underset{\_}{s}}^{\prime} \cdot \text{?}} = {{{- \sin}\; 2{\phi \left( {{\text{?}\cos \; \phi} + {{\frac{k}{\phi} \cdot {- \sin}}\; 2\phi} + {2{k \cdot {- \cos}}\; 2\phi}} \right)}} +}} \\ {{{{+ \cos}\; 2{\phi \left( {{\text{?}\sin \; \phi} + {{\frac{k}{\phi} \cdot \cos}\; 2\phi} + {2{k \cdot {- \sin}}\; 2\phi}} \right)}} =}} \\ {= {{\text{?} \cdot \underset{\underset{= {{\sin {({{{- 2}\phi} + \phi})}} = {- {\sin {(\phi)}}}}}{}}{\left( {{{- \sin}\; 2{\phi cos}\; \phi} + {\cos \; 2{\phi sin}\; \phi}} \right)}} +}} \\ {{{\frac{k}{\phi}\underset{\underset{1}{}}{\left( {{\sin^{2}2\phi} + {\cos^{2}2\phi}} \right)}} + {2{k \cdot \underset{\underset{\varphi}{}}{\left( {{\sin \; 2{\phi cos}\; 2\phi} - {\sin \; 2{\phi cos}\; 2\phi}} \right)}}}}} \end{matrix}$ $\mspace{20mu} \underset{\underset{\_}{\_}}{{{\underset{\_}{s}}^{\prime} \cdot \text{?}} = {\frac{k}{\phi} - {\text{?}\sin \; \phi}}}$ ?indicates text missing or illegible when filed

and that

${{\underset{\_}{s}}^{''} \cdot \text{?}} = {\frac{1}{\sqrt{\ldots}} \cdot \left\{ {\left( {{\text{?}\cos \; \phi} + \frac{k}{\phi} - {\sin \; 2\phi} + {2{k \cdot {- \cos}}\; 2\phi}} \right){\left( {\text{?} + {\text{?}\text{?}} + {{k \cdot {- \sin}}\; 2\phi}} \right)++}\left( {{\text{?}\sin \; \phi} + {{\frac{k}{\phi} \cdot \cos}\; 2\phi} + {2{k \cdot {- \sin}}\; 2\phi}} \right)\left( {\text{?} - \text{?} + {{\text{?} \cdot {- \cos}}\; \phi} + {k\; \cos \; 2\phi}} \right)} \right\}}$ ?indicates text missing or illegible when filed

if the values obtained above for s′·t _(SM)(φ) and s″·t _(SM)(φ) are inserted into the equation s′·t _(SM)(φ)=s″·t _(SM)(φ) and this is resolved according to dk/dφ, an elliptical expression is produced owing to the

which can only be integrated analytically in a complex manner.

Accordingly, the function obtained above

$\mspace{20mu} {\frac{k}{\phi} = {\text{?}\left( {\text{?}\text{?}} \right)}}$ ?indicates text missing or illegible when filed

is defined numerically in another further preferred embodiment as K=k(φ). The person skilled in the art can do this easily using the program mathcad from the company mathsoft.

The values obtained for k(φ) can then be inserted into SM(φ), and the shape of the secondary concentrator 40,65 can thereby be defined by a line of points. For example, the method of least squares then produces a continuous shape, which comes closer and closer to the ideal shape as the number of defined points SM(φ) increases (or the interval for Δφ is reduced).

The person skilled in the art can now select whether the above-mentioned line of points is defined with a larger or smaller Δφ, that is, whether the points lie closely or less closely together, so that a focal line region or an exact focal line is produced at the location of the absorber pipe 42,64. This means that in the actual case the temperature distribution on the absorber pipe which is described above in connection with FIG. 3 can be influenced in the desired sense.

Owing to the diameter of the solar disc, the solar rays 31 do not only reach the concentrator 32,62 in parallel but with a beam divergence of approx. 0.5 degrees. Accordingly, the above-described method can be used e.g. to define the shape of the secondary concentrator 40,65 in each case for a unit vector s, which runs parallel to the y axis, and for a unit vector s, which is inclined by 0.25 degrees to the left and right of the y axis. There are then produced three shapes for the secondary concentrator 40,65, or a region in which its surface should lie, in which an optimization can be carried out.

In a preferred embodiment, the incident rays are for the purpose of simplification assumed to be parallel, with them lying particularly preferably parallel to the center plane of symmetry of the concentrator 32,62.

The method specified above allows the unit vector s′ to be specified even with a concentrator which is not circular arc-shaped, for example if it is constructed in a parabolic manner. In a further preferred exemplary embodiment, however, the concentrator 32,62 is assumed to have an arcuate cross section.

In another further preferred embodiment of the production method according to the invention, the origin of the co-ordinate system is assumed to be the center point of curvature C of the concentrator 32,62.

If the shape of the reflective surface of the secondary concentrator 40,65 is defined once, it can be transferred to the secondary concentrator, preferably to a block of suitable foam which forms the body of the secondary concentrator, after which the surface, which has been processed according to the curve, of the foam is covered with a reflective foil, and a secondary concentrator 40,65 which can be built into the trough collector is thus produced. Such foam blocks particularly preferably have a uniform length of for example 2 m and are then installed in an assembled manner for the trough collectors to be fitted.

A ready-to-use secondary concentrator 40,65 is thus produced, which is matched to a predefined concentrator 32,62 and to a predefined absorber pipe 42,64. Although such a secondary concentrator 40,65 is particularly suitable for installation into a pressure cell, it can of course also be used in conventional trough collectors without a pressure cell (e.g. with regard to the above-mentioned shortened ray path). The method according to the invention can likewise be used e.g. in dish/Stirling collectors with round (and not trough-shaped) concentrator if the radiation is to be concentrated through a hole in the concentrator onto a point which lies behind the concentrator. 

1. A trough collector for a solar power plant comprising: a concentrator arranged in a pressure cell such that it can be loaded with pressure and constructed as a flexible membrane; wherein the concentrator concentrates solar radiation incident into the pressure cell under operating pressure towards a focal line region running outside of the pressure cell, and with an absorber element for absorption of a concentrated solar radiation; wherein the absorber element is arranged in the pressure cell; and wherein further within the pressure cell a secondary concentrator is provided which lies in a path of a concentrating radiation and is constructed in such a manner that the concentrated radiation is further concentrated toward a secondary focal line region or a secondary focal line at a location of the absorber element.
 2. The trough collector as claimed in claim 1, wherein: the pressure cell at least partially formed from a flexible membrane has a transparent region for solar radiation to be reflected; the concentrator comprises: a side which faces the transparent region and reflects the solar radiation; and a side which faces away from the radiation; wherein the concentrator subdivides the pressure cell into a first pressure chamber and a second pressure chamber; and wherein means constructed as fans are provided to operationally generate and maintain pressure at a predefined level in the first and second pressure chambers if a volume of one or both pressure chambers is changed in operation by means of external influences.
 3. The trough collector as claimed in claim 1, wherein a predefined pressure difference is less than 0.5 mbar, preferably in a range from 0.05 to 0.2 mbar, and particularly preferably in the range from 0.05 to 0.1 mbar.
 4. The trough collector as claimed in claim 1, wherein: the secondary concentrator is arranged opposite the concentrator in a vicinity of a membrane with a region which is transparent for solar radiation; and wherein the absorber element runs between the concentrator and the secondary concentrator.
 5. The trough collector as claimed in claim 1, wherein: the absorber element comprises a jacket for reducing heat emission to the outside; wherein the absorber element further comprises a thermal opening running longitudinally in the jacket; wherein the thermal opening allows passage of solar radiation further concentrated by the secondary concentrator through the jacket into an interior of the absorber element; and wherein the absorber element is further constructed in such a manner that a heat-transporting fluid can circulate between the jacket and the interior for transporting radiant heat incident through the thermal opening away.
 6. The trough collector as claimed in claim 1, wherein the concentrator and the secondary concentrator are each divided into two longitudinally running mutually symmetrical regions. 7-9. (canceled)
 10. A method for producing a secondary concentrator for a solar collector, which comprises the following steps for defining a shape of a reflective surface of the secondary concentrator: taking at least one cross-sectional plane of the secondary concentrator, wherein the following is carried out therein: defining a unit vector s(φ) of an incident solar ray; defining the unit vector s′(φ) of the solar ray reflected on the concentrator; taking the unit vector s″(φ) of the solar ray reflected on the secondary concentrator; defining a number of points SM(φ) on the secondary concentrator by means of a vector PM(φ), wherein the vector PM(φ) lies on the concentrator, and where SM(φ)=PM(φ)+k·s′(φ); defining a factor k in such a manner that s″(φ) goes through a predefined focal point or a focal line F of the arrangement consisting of the concentrator and the secondary concentrator; defining a curve, and thus the sought shape of the reflective surface which goes through the obtained number of points SM(φ); and transferring the curve obtained using the above steps onto the secondary concentrator to form its reflective surface.
 11. The method as claimed in claim 10, wherein the factor k is defined by an equalization of scalar products s′·t (φ)= s″·t (φ) wherein t(φ) is a tangent which is placed on the secondary concentrator at the point SM(φ).
 12. The method as claimed in claim 11, wherein the equation s′·t (φ)= s″·t (φ) is resolved according to dk/dφ, wherein the values of k are defined numerically from a resulting elliptical integral.
 13. The method as claimed in claim 10, wherein the incident solar rays are assumed to be parallel for simplification.
 14. The method as claimed in claim 10, wherein the concentrator is assumed to have an arcuate cross section.
 15. The method as claimed in claim 10, wherein an origin of the co-ordinate system is assumed to lie in a center point of curvature of the concentrator.
 16. The method as claimed in claim 10, wherein the curve is defined by means of the method of least squares by means of the points SM(φ) obtained.
 17. The method as claimed in claim 10, wherein the curve is transferred to a foam block and the produced curved surface is covered with a reflective foil.
 18. (canceled)
 19. A trough collector for a solar power plant comprising the following steps for defining a shape of a reflective surface of a secondary concentrator: taking at least one cross-sectional plane of the secondary concentrator, wherein the following is carried out therein: defining a unit vector s(φ) of an incident solar ray; defining the unit vector s′(φ) of the solar ray reflected on the concentrator; taking the unit vector s″(φ) of the solar ray reflected on the secondary concentrator; defining a number of points SM(φ) on the secondary concentrator by means of a vector PM(φ), wherein the vector PM(φ) lies on the concentrator, and where SM(φ)=PM(φ)+k·s′(φ); defining a factor k in such a manner that s″(φ) goes through a predefined focal point or a focal line F of the arrangement consisting of the concentrator and the secondary concentrator; defining a curve, and thus the sought shape of the reflective surface which goes through the obtained number of points SM(φ); and transferring the curve obtained using the above steps onto the secondary concentrator to form its reflective surface.
 20. (canceled) 